Rab geranylgeranyl transferase (GGT) catalyzes the reaction that attaches the 20 carbon geranylgeranyl group to the C-terminal cystein motifs of a family of small Ras-like GTPase, the Rab proteins. Rab GGT comprises two tightly associated ? and _ subunits of 65 kD and 38 kD, respectively. We have obtained single GGT crystals in the P1 space group which diffract to 2.5 E resolution in our laboratory. So far we have identified two heavy atom derivatives, but due to the limitation of the P1 space group, we were unable to collect the complete anomalous data set on these derivatives in our laboratory. We believe that tunable X-ray source at synchrotron and rapid data collection with the CCD detector will enable us to make use of the anomalous signals from these derivatives for phasing, and lead to the final solution of the GGT structure.